1. Field of the Invention
This invention relates to a communications system and to a method of communicating.
2. Discussion of Prior Arts
Communications systems are well known in the prior art. Those employing infrared radiation as the carrier signal permit the elimination of special transmitting media such as wires without being subject to the broadcasting regulations of radio frequency transmission systems. The first capability offers significant advantages in applications for which space, presentation or frequent reconfiguration are essential requirements. The second makes infrared communication an attractive alternative to radio communication. Generally infrared communication systems can be loosely divided into those in which unauthorised interception of information is unimportant (e.g. those based on an extension of a television remote control principle) and those in which it could have serious consequences (e.g. secure communications).
European Patent Application 85103928.9 discloses an infrared communications system providing wireless communication. It is specifically applicable to interactions between a computer terminal and peripheral devices such as keyboards, displays and printers and readily extendible to computer interactions in a local network. Similar advantages are to be found in shop-based systems enabling communication between point of sale (POS) terminals and a central unit for stock control or other purpose. Such a system is described in UK Patent Application GB 2 222 335. In both systems emphasis is placed on how to make the signal distinguishable above background infrared radiation rather than on how to prevent deliberate interference with or detection of the signal.
The latter category of secure communications has important applications in transmission of both commercially and militarily sensitive information.
As an alternative to the complex encoding procedures employed in some secure communication schemes, GB 776 129, filed in 1955, discloses a technique whereby electrical signals are encoded onto optical carriers such that the amplitude of the transmitted signal is constant. Contemporary optical receivers typically employed circuits whose time constants were appreciable. They thus integrated the rapid fluctuations of intensity employed in the encoded signal and merely reproduced the envelope, namely a dc voltage or current of constant strength. However GB 776 129 does not disclose a truly secure communication. The presence of a signal can be detected, albeit that the signal is not readily interpreted.
Variations about a constant intensity are also disclosed in GB 2 157 523 A. In this system the carrier radiation propagates along an optical fibre encoded with signals arranged to have a substantially constant mean intensity. Breakage or unauthorised tapping along the fibre optic line results in a variation of the mean intensity at reception. By this means, unauthorised disturbance can be detected. Again, however, this does not mask the presence of a signal.
UK Patent Application GB 2 041 703 employs a technique of translating serial digital data into ac voltage pulses each of which has a time integral of zero. It discloses the application of such an ac voltage for data storage on a magnetic medium such as a magnetic tape. The use of a signal whose time integral is zero reduces the likelihood of a net residual flux being produced at the magnetic recording head. This therefore lessens the chance of permanent head magnetisation and consequent performance decline.
Thus conventional secure communication systems suffer from the disadvantage that it is difficult to mask the entire signal. This may betray the presence of a transmitter and possibly prompt decoding activities by unauthorised detectors.